The time between both logs "PATH IN PHONE" and "PATH IN FIREBASE" is around a minute the first time, then is quite fast. The question is, do you know a way or method or pattern to have the speed upload all the time and not just the second time?
Or what do you suggest? is it better store the image in Base64 using the realtime database instead of storage?

The reason you see the lag on your first upload is because Firebase is backed by Google App Engine which when your backend is not in use for a long period of time basically puts it in a paused state to save resources.

When you go to make a request when it is in this paused state it needs to spin back up everything which is when you see the lag in upload time